1. Field of the Invention
The present invention relates to a radio receiving apparatus and a radio receiving method.
2. Description of the Related Art
When radio communication is performed, radio waves that reach from a transmission antenna to a reception antenna include, besides a direct wave that directly reaches from the transmission antenna, various indirect waves such as a reflected wave that reaches the reception antenna after reflecting on an object present around the transmission antenna. A receiver detects a signal of interference between the direct wave and the indirect waves. However, a form of the interference is different depending on a frequency of a radio wave and fluctuates with time according to the movement of the object present around the transmission antenna.
For example, in orthogonal frequency division multiplex (OFDM) (hereinafter, “OFDM communication”) for performing signal transmission using a plurality of sub-carriers obtained by modulating a plurality of carrier waves orthogonal to one another, the interference by the direct wave and the indirect waves appears as shifts of the amplitudes and the phases of the carrier waves. The shifts of the amplitudes and the phases are referred to as transmission channel characteristics. For demodulation of an OFDM signal, it is necessary to estimate the transmission channel characteristics for the respective carrier waves.
On the other hand, usually, a known signal is present at the head of an OFDM packet. The receiver performs transmission channel estimation using the known signal and demodulates the remaining data section (a section other than the known signal) of the packet. When an element of time fluctuation is not present in the transmission channel, only this processing has to be performed. However, when an element of time fluctuation is present in the transmission channel, actual transmission channel characteristics gradually shifts from an estimation result of the head of the packet. Finally, the demodulation cannot be performed. Therefore, when a long OFDM packet is received or in mobile communication with intense transmission channel fluctuation, processing for tracking the transmission channel (transmission channel tracking processing) is necessary.
In processing considered to be effective as the transmission channel tracking processing, when a transmitter creates a transmission packet, for example, the known signal also inserted in the packet to allow the receiver to estimate a transmission channel. Actually, in a relatively new radio communication standard, this kind of consideration is paid. For example, in a radio communication standard called mobile WiMAX (IEEE802.16e), a technology for inserting a pilot sub-carrier for transmission channel estimation in a packet is adopted. On the other hand, in a relatively old standard such as IEEE802.11a, such consideration is not paid. There is no other way but to track the transmission channel fluctuation without using the known signal.
As a related art for performing the transmission channel tracking without using the known signal, for example, there is Japanese Patent Application Laid-Open No. 2002-44049 (hereinafter, “Patent Document 1”). In Patent Document 1, hard determination or soft determination is applied to a signal subjected to channel equalization processing, remaining frequency error correction processing, and sampling frequency error correction processing of reception processing to calculate a signal point during transmission as a reference signal. After the reference signal is calculated, the reference signal is compared with an actual reception signal point to calculate shifts of amplitude and a phase and perform various kinds of correction.
However, in the processing disclosed in Patent Document 1, pre-processing for converting a reception component into polar coordinates of amplitude and a phase is necessary in arithmetic processing for an amplitude ratio to the reference signal and arithmetic processing for a phase difference. An increase in a calculation amount is inevitable. Therefore, when the technology disclosed in Patent Document 1 is adopted, power consumption and a circuit size increase because of the increase in the calculation amount.